Lateral variation of major and trace elements in the Pliocene volcanic rock of the rock of the Northeast Japan.

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It has been well-known that the Quaternary volcanic rocks increase gradually in alkali contents, especially in K_2O, other large ion lithophile elements (LIL elements), and high field strength elements (HFS elements) from the volcanic front toward the back-arc side across the NE Japan arc (Kuno, 1960; Kawano et al., 1961; Masuda, 1979; Fujimaki and Kurasawa, 1980; Fujitani and Masuda, 1981; Yoshida and Aoki, 1984; Sakuyama and Nesbitt, 1986; Nakagawa et al., 1988). More recently, several workers suggested the existence of the lateral variation of LIL elements including K, Rb and Sr of the volcanic rocks during the late Miocene time, based on the geochemical investigations of the Tertiary volcanic rocks from both the central Honshu and northern Honshu to southwest Hokkaido (Shuto et al., 1988; Tamura and Shuto, 1989; Okamura, 1987).In the NE Japan arc, the volcanic rocks formed by the Pliocene volcanism from 4 to 2 Ma show a widespread distribution which coincides roughly with that of the Quaternary volcanic rocks (Fig.1). We examined the abundances of major and trace elements for 111 rock samples from twelve Pliocene volcanic centers in the NE Japan arc. Based on these results and the published analyses, an across-arc compositional variation for the Pliocene volcanics will be examined in comparison with that for the Quaternary volcanics in the NE Japan arc.